Lately, it is strongly demanded to reduce a fuel consumption of an automobile and hence a tire having a low rolling resistance is required. For this end, there is required a rubber composition having a low tan δ (hereinafter referred to as a low loss factor) and being excellent in a low heat buildup as a rubber composition used in a tread or the like of the tire. Also, the rubber composition for the tread is required to be excellent in a wear resistance and fracture characteristics in addition to the low loss factor.
In order to improve the low loss factor, wear resistance and fracture characteristics of the rubber composition formed by compounding carbon black into a rubber component, it is effective to improve an affinity between the carbon black and the rubber component in the rubber composition. For example, in order to improve the reinforcing effect with carbon black by improving the affinity between the carbon black and the rubber component in the rubber composition, there are developed a synthetic rubber wherein the affinity for carbon black is improved by a terminal modification, a synthetic rubber wherein the affinity for carbon black is improved by copolymerizing with a functional group-containing monomer and so on.
Also, tan δ of the rubber composition can be lowered to reduce the heat buildup by compounding an inorganic filler such as silica or the like into the rubber component. However, since the inorganic filler is generally low in the affinity for the rubber component, the sufficient reinforcing property cannot be obtained and hence the wear resistance and fracture characteristics of the rubber composition are deteriorated. On the contrary, in order to improve the reinforcing effect with the inorganic filler by improving the affinity between the inorganic filler and the rubber component in the rubber composition, there are developed a synthetic rubber wherein the affinity for the inorganic filler is improved by a terminal modification, a synthetic rubber wherein the affinity for the inorganic filler is improved by copolymerizing with a functional group-containing monomer and so on.
On the other hand, a natural rubber is voluminously used while utilizing its excellent physical characteristics, but there is no technique wherein the affinity for the carbon black or the inorganic filler is improved by modifying the natural rubber itself to highly improve the reinforcing effect with the filler.
For example, there is proposed a technique for epoxidizing the natural rubber. In this technique, however, the affinity between the natural rubber and the filler cannot be sufficiently improved, so that the reinforcing effect with the filler cannot be sufficiently improved. Also, there is known a technique wherein a graft-polymerization is conducted by adding a vinyl-based monomer to a natural rubber latex (see JP-A-H05-287121, JP-A-H06-329702, JP-A-H09-25468, JP-A-2000-319339, JP-A-2002-138266 and JP-A-2002-348559). The grafted natural rubber obtained by this technique is put into a practical use as an adhesive or the like. In the grafted natural rubber, however, a large amount of the vinyl compound as a monomer (20-50% by mass) is grafted for changing the characteristics of the natural rubber itself, so that when it is compounded with a filler, the viscosity is largely increased and the processability is deteriorated. Moreover, since the large amount of the vinyl compound is introduced into the molecular chain of the natural rubber, there are damaged the excellent physical characteristics inherent to natural rubber (viscoelasticity, stress-strain curve in a tensile test and the like).
On the other hand, as a technique for improving a dispersibility of a filler into natural rubber is known a method for producing a natural rubber masterbatch by mixing a natural rubber latex with a slurry solution formed by previously dispersing the filler into water. However, a rubber composition using the natural rubber masterbatch is not sufficient in the reinforcing property, so that there is still a room for improving the wear resistance and fracture characteristics.